(Interesting stuff about how WP builds the feed each time it is requested, good to know, thanks Otto.)
So if I’m understanding correctly, it’s up to the feed reader to “notice” that a post has been updated and indicate that in its UI.
When I look at the XML code for my feed, it is showing my updated post text and time-stamp. So WordPress is behaving as intended.
I’m using an RSS reader app called SharpReader, and I don’t know if it’s caching the old request, or just doesn’t think the updated post should be shown as new, I’ll play with it and see where I get.
The key is that the problem isn’t with WordPress. Thanks everyone!
